1. Field of the Invention
The present invention relates to an absorbent composition and to a process for the selective absorption of one or more gaseous acidic component(s) from a normally gaseous mixture containing said gaseous acidic component(s) and gaseous non-acidic components and CO2—using the absorbent composition.
2. Description of the Related Art
It is well known in the art to treat gases and liquids, such as mixtures containing acidic gases including CO2, H2S, CS2, HCN, COS and oxygen and sulfur derivatives of C1 to C4 hydrocarbons with amine solutions to remove these acidic gases. The amine usually contacts the acidic gases and the liquids as an aqueous solution containing the amine in an absorber tower with the aqueous amine solution contacting the acidic fluid countercurrently.
The treatment of acid gas mixtures containing, inter alia, CO2 and H2S with amine solutions typically results in the simultaneous removal of substantial amounts of both the CO2 and H2S. For example, in one such process generally referred to as the “aqueous amine process”, relatively concentrated amine solutions are employed. A recent improvement of this process involves the use of sterically hindered amines as described in U.S. Pat. No. 4,112,052, to obtain nearly complete removal of acid gases such as CO2 and H2S. This type of process may be used where the partial pressures of the CO2 and related gases are low. Another process often used for specialized applications where the partial pressure of CO2 is extremely high and/or where many acid gases are present, e.g., H2S, COS, CH3SH and CS2 involves the use of an amine in combination with a physical absorbent, generally referred to as the “nonaqueous solvent process”. An improvement on this process involves the use of sterically hindered amines and organic solvents as the physical absorbent such as described in U.S. Pat. No. 4,112,051.
It is often desirable, however, to treat acid gas mixtures containing both CO2 and H2S so as to remove the H2S selectively from the mixture, thereby minimizing removal of the CO2. Selective removal of H2S results in a relatively high H2S/CO2 ratio in the separated acid gas which simplifies the conversion of H2S to elemental sulfur using the Claus process.
The typical reactions of aqueous secondary and tertiary amines with CO2 and H2S can be represented as follows:H2S+R3N⇄R3NH++SH−  (1)H2S+R2NH⇄R2NH2++SH−  (2)CO2+R3N+H2O⇄R3NH++HCO3−  (3)CO2+2R2NH⇄R2NH2++R2NCOO−  (4)RNH2+CO2⇄RN+H2CO2−  (5)RN+H2CO2−+RNH2⇄RNHCO2−RNH3+  (6)wherein each R is an organic radical which may be the same or different and may be substituted with an hydroxy group. The above reactions are reversible, and the partial pressures of both CO2 and H2S are thus important in determining the degree to which the above reactions occur.
While selective H2S removal is applicable to a number of gas treating operations including treatment of hydrocarbon gases from shale pyrolysis, refinery gas and natural gas having a low H2S/CO2 ratio, it is particularly desirable in the treatment of gases wherein the partial pressure of H2S is relatively low compared to that of CO2 because the capacity of an amine to absorb H2S from the latter type gases is very low. Examples of gases with relatively low partial pressures of H2S include synthetic gases made by coal gasification, sulfur plant tail gas and low-Joule fuel gases encountered in refineries where heavy residual oil is being thermally converted to lower molecular weight liquids and gases.
Although it is known that solutions of primary and secondary amines such as monoethanolamine (MEA), diethanolamine (DEA), dipropanolamine (DPA), and hydroxyethoxyethylamine (DGA) absorb both H2S and CO2 gas, they have not proven especially satisfactory for preferential absorption of H2S to the exclusion of CO2 because the amines undergo a facile reaction with CO2 to form carbamates as shown in Equations 5 and 6.
Diisopropanolamine (DIPA) is relatively unique among secondary aminoalcohols in that it has been used industrially, alone or with a physical solvent such as sulfolane, for selective removal of H2S from gases containing H2S and CO2, but contact times must be kept relatively short to take advantage of the faster reaction of H2S with the amine compared to the rate of CO2 reaction shown in Equations 2 and 4 hereinabove.
In 1950, Frazier and Kohl, Ind. and Eng. Chem., 42, 2288 (1950) showed that the tertiary amine, methyldiethanolamine (MDEA), has a high degree of selectivity toward H2S absorption over CO2. This greater selectivity was attributed to the relatively slow chemical reaction of CO2 with tertiary amines as compared to the rapid chemical reaction of H2S. The commercial usefulness of MDEA, however, is limited because of its restricted capacity for H2S loading and its limited ability to reduce the H2S content to the level at low pressures which is necessary for treating, for example, synthetic gases made by coal gasification.
Recently, U.K. Patent Publication No. 2,017,524A to Shell disclosed that aqueous solutions of dialkylmonoalkanolamines, and particularly diethyl-monoethanolamine (DEAE), have higher selectivity and capacity for H2S removal at higher loading levels than MDEA solutions. Nevertheless, even DEAE is not very effective for the low H2S loading frequency encountered in the industry. Also, DEAE has a boiling point of 161° C., and as such, it is characterized as being a low-boiling, relatively highly volatile amino alcohol. Such high volatilities under most gas scrubbing conditions result in large material losses with consequent losses in economic advantages.
U.S. Pat. Nos. 4,405,581; 4,405,583 and 4,405,585 disclose the use of severely sterically hindered amine compounds for the selective removal of H2S in the presence of CO2. Compared to aqueous methyldiethanolamine (MDEA) severely sterically hindered amines lead to much higher selectivity at high H2S loadings.
U.S. Pat. No. 4,487,967 discloses a catalytic synthesis process for selectively preparing severely sterically hindered secondary aminoether alcohols by reacting a primary amino compound with a polyalkenyl ether glycol in the presence of a hydrogenation catalyst at elevated temperatures and pressures.
U.S. Pat. No. 4,665,195 discloses a catalytic synthesis process for producing di-amino-polyalkenyl ethers by reacting (a) one or more acyclic or heterocyclic amino compounds with (b) one or more polyalkenyl ether glycols or polyalkenyl amino ether alcohols, in the presence of a hydrogenation catalyst at elevated temperatures and pressures.
The composition of BTEE has been disclosed in U.S. Pat. No. 4,405,583 and synthesized from tertiary-butylamine and bis-(2-chloroethoxy)-ethane. However, an aqueous BTEE solution suffered from phase separation under regeneration conditions (about 110° C.). EEETB is disclosed as a new composition of matter in U.S. Pat. No. 4,471,138 and can be prepared from tertiary-butylamine and chloroethoxyethoxyethanol. EEETB in aqueous solution can be used for the selective removal of H2S in the presence of CO2. However, the BTEE/EEETB mixture gives a better selectivity and a higher capacity for H2S than EEETB. The mixture does not have phase separation under regeneration conditions, i.e., this mixture overcomes the phase separation problem of BTEE. The BTEE/EEETB mixture also gives higher selectivities for H2S than observed with the severely sterically hindered amines, e.g., ethoxyethanol-tertiary-butylamine (EETB), described in U.S. Pat. Nos. 4,405,581 and 4,405,585.
U.S. Pat. No. 4,417,075 teaches a class of di-secondary amino ethers of the formula
wherein R1 and R8 are each independently selected from the group consisting of primary alkyl having 1-8 carbon atoms, and primary hydroxy alkyl having 2-8 carbon atoms, secondary alkyl and secondary hydroxy alkyl radicals having 3-8 carbon atoms, tertiary alkyl and tertiary hydroxy alkyl radicals having 4 to 8 carbon atoms, R2, R3, R4, R5, R6 and R7 are each independently selected from the group consisting of hydrogen, C1-C3 alkyl and hydroxyalkyl radicals, with that proviso that R2, R3, R6 and R7 are C1-C4 alkyl or hydroxy alkyl radicals when R1 and R8 are primary alkyl or hydroxy alkyl radicals and at least one of R2 or R3 and R6 and R7 are C1 to C3 alkyl or hydroxyalkyl radicals when R1 and R8 are secondary alkyl radicals, m, n and p are positive integers ranging from 2 to 4 and a is either zero or a positive integer ranging from 1 to 10. These compounds are useful in the selective removal of H2S from gaseous mixtures containing H2S and CO2.
U.S. Pat. No. 4,894,178 teaches a mixture of two severely hindered amines with the following formula:
with x being an integer ranging from 2 to 6, and the weight ratio of the first amine to the second amine ranging from 0.43:1 to 2.3:1. This mixture can be prepared in the one-step synthesis, by the catalytic tertiary-butylamination of the polyalkenyl ether. glycol, HO—(CH2CH2O)x—CH2CH2—OH. For example, a mixture of bis(tertiary-butylaminoethoxy)ethane (BTEE) and ethoxyethoxyethanol-tertiary-butylamine (EEETB) can be obtained by the catalytic tertiarybutylaminaton of triethylene glycol.